1. Field of the Invention
The present invention relates to means for eliminating multipacting in an RF/microwave component, and particularly to such a component wherein the RF potential is rectified to derive a dc potential which self-biases the component so as to prevent polarity reversal across the component electrodes, thereby to eliminate multipacting.
2. Description of the Prior Art
The multipacting mechanism of RF voltage breakdown in RF/microwave components is of considerable importance when such components are used in very high vacuum environments such as those encountered in space. A good description of the multi-pacting mechanism is set forth in the Jet Propulsion Laboratory Technical Report 32-1500 entitled "Final Report on RF Voltage Breakdown in Coaxial Transmission Lines" by R. Woo.
At very low pressures, such as those encountered in space, the mean free path of electrons may be longer than the separation distance d between electrodes of the microwave component. Under these conditions, electrons can readily travel between the component electrodes without undergoing collisions with the gas molecules. If an electron should collide with one of the electrodes, secondary electrons may be released. If this occurs as the RF electric field within the component passes through zero, the reverse electric field will accelerate the electrons back across the gap between the component electrodes. If the transit time of the electrons across the gap is one-half cycle of the RF field, the secondary electrons formed by the initial collision become primary electrons when they strike the other component electrode and cause the release therefrom of additional secondary electrons. These in turn are accelerated toward the first electrode by the now reversed polarity of the RF electric field.
This operation repeats itself each half-cycle of the RF field, resulting in the rapid build-up of large electron densities within the gap between the microwave component electrodes. Rf voltage breakdown results. At the onset of such multipacting breakdown, as much as seventy-five percent of the RF energy introduced into the component may be lost. Should such multipacting breakdown continue, typically twenty percent or more of the introduced RF energy will be dissipated. The "lost" energy is expended in the undesired secondary electron production, and often results in excessive heating of the microwave component electrode. In extreme cases, the component may become so hot as to glow.
Woo (Op.Cit.) and others have found that the RF potential or breakdown voltage at which multipacting occurs may be very low. Indeed, they have found that this breakdown voltage is a function of the product F.multidot.d of the frequency F of operation of the component times the gap spacing d between the component electrodes. FIG. 1 shows a typical graph of this relationship (and is adapted from the Woo paper cited above). By way of example, for an RF/microwave component operating at 100 MHz and an electrode gap spacing of 1 cm (so as to have an F.multidot.d product of 100), multipacting breakdown will occur with RF potentials as low as forty volts. Indeed, as shown by the broken line in FIG. 1, multipacting breakdown will occur for this configuration with RF potentials between about forty volts and one hundred volts.
Reducing or increasing the electrode gap spacing d so that the product F.multidot.d lies outside the multi-pacting region (FIG. 1) is one approach to eliminating the multipacting problem. However, such dimension parameters (i.e., reduction or increase in the electrode gap spacing) may be impractical or undesirable in the particular RF/microwave component design. Therefore this approach to elimination of the multipacting problem may be disadvantageous for a particular component.
A more generally acceptable technique for elimination of multipacting is to superimpose a dc bias across the component electrodes. If this bias is sufficiently large, then the RF electric field potential, when superimposed on the dc bias, will always be of the same polarity. For example, if the RF electric field potential varies between +40 V and -40 V, but a dc bias of +50 V is applied across the component electrodes, the net electric field across the gap will vary between +10 V and +90 V.
The effect of this dc bias is to completely eliminate the multipacting. Since the polarity of the electric field between the component electrodes does not change each cycle of the RF field, then the condition necessary for accelerating the secondary emission electrons across the gap is missing. In other words, since the field polarity does not reverse, there will be no acceleration of electrons in alternating directions across the electrode gap on alternate half-cycles of the RF field. The basic condition necessary for multipacting is no longer present.
In the past, such dc biasing has been provided by a power supply external to the microwave components itself. Although effective in eliminating multipacting, the requirement for providing this external dc biasing power supply has added significantly to the cost, size and weight of the microwave assembly. It is this shortcoming of the prior art which is overcome by the present invention, an objective of which is to provide an RF/microwave component which is self-biasing. A concomitant objective is to provide a means for self-biasing an RF/microwave component so as to obviate the need for a separate dc biasing power supply to eliminate multipacting.
A further objective is to provide an RF/microwave component which includes means for deriving a dc bias directly from the RF potential developed across the component electrodes, thereby eliminating the need for an external dc biasing power supply.